1. Field of the Invention
The present invention relates to a thin film semiconductor device having a polycrystalline semiconductor film, and a process and apparatus for producing a polycrystalline semiconductor film. The thin film semiconductor device of the present invention is useful for image display devices.
2. Related Art
The prior art process for crystallizing an amorphous silicon thin film by scanning with a pulse laser will be described with reference to FIG. 12. FIG. 12 shows the most general process for crystallizing with an excimer pulse laser of the prior art. The whole substrate is crystallized by irradiating a non-crystal silicon film 102 which was formed on a substrate 100 through an base film 101, with a laser beam 105 from a linear excimer laser having a width L of several millimeters on the substrate and moving the laser exposure position at intervals of 1 to several pulses. In this prior art process, crystal nuclei are formed at random upon laser exposure. In addition, the average distance between the formed crystal nuclei is 0.5 xcexcm or less under ordinary laser annealing conditions. Therefore, the obtained polycrystalline silicon film 103 has a grain size of 0.5 xcexcm or less and is not uniform in size.
Further, an international patent publication WO9745827 discloses the following process. That is, when the width L of the laser beam 105 shown in FIG. 12 is reduced to 0.5 xcexcm or less and the position of the laser beam 105 having this shape is moved 0.5 xcexcm or less each time to irradiate pulses, crystal grows in one direction with the initially formed crystal grains as seeds. The above one direction is a transverse direction, that is, a direction perpendicular to the thickness direction of the grown film.
In the above process of the prior art, as the time required for crystal growth is 100 ns or less, the obtained crystals have a grain size of 1 xcexcm or less and are greatly nonuniform in grain size. The orientation of grains is out of order, the density of defects is large, and the roughness of the film surface is large. Therefore, it is impossible to grow polycrystalline silicon having a large grain size or to control the grain size or the position of the grain boundary accurately. Therefore, the grain boundary is included in the channel at random. Consequently, it is difficult to improve the characteristic properties, reliability and uniformity of TFT devices.
Since the beam must be converged to a size of 1 xcexcm or less in the technology disclosed by the above international patent publication WO9745827, the energy of the laser is lost and the optical system of an irradiation laser become complicated. As the moving distance between laser pulses is 1 xcexcm or less, it takes a long time to crystallize the whole substrate and it is difficult to improve throughput and reduce costs. Particularly, this process cannot be applied to a large-area substrate. Further, very small distance movement is easily influenced by vibration and involves a yield problem.
It is a first object of the present invention to provide a production process and apparatus for forming on an insulating substrate made from glass or the like a high-quality polycrystalline semiconductor film whose grain boundary, grain size and crystal orientation can be controlled and whose film roughness and crystal defects formed in crystallization process have been reduced, and to provide a thin semiconductor device comprising the above polycrystalline semiconductor film.
It is a second object of the present invention to provide a production process and apparatus for forming a low-cost and high-quality polycrystalline semiconductor film, which can reduce the number of production steps, can be applied to a large-area substrate and have a high throughput, and to provide a thin film semiconductor device comprising the above polycrystalline semiconductor film.
It is a third object of the present invention to provide a production process and apparatus for forming on an inexpensive insulating substrate made from glass or the like a high-quality polycrystalline semiconductor film which operates with high performance and high reliability and is excellent in uniformity among devices, and to provide a thin film semiconductor device comprising the above polycrystalline semiconductor film.
The major aspects of the present invention will be described below.
According to a first aspect of the present invention, there is provided a thin film semiconductor device which has an insulating substrate, a first semiconductor film which is a polycrystalline semiconductor film, a gate electrode formed on the first semiconductor film through a gate insulating film, first charge transmitting and receiving means and second charge transmitting and receiving means formed on the first semiconductor film at a predetermined interval therebetween, and a channel region formed between the first and second charge transmitting and receiving means, wherein
the main orientation of the first semiconductor film constituting the channel region is {110} with respect to the main surface of the insulating substrate or the gate insulating film.
It is possible to provide a thin film semiconductor device having high reliability by selecting the main orientation of the semiconductor film of the channel region with respect to the main surface of the insulating substrate or the gate insulating film even when a polycrystalline semiconductor film is used. The method of controlling the main orientation of the polycrystalline film will be described hereinafter.
An MIS type thin film semiconductor device according to the present invention may vary in structure as follows.
(1) An MIS type thin film semiconductor device having a gate electrode which is formed on a predetermined polycrystalline semiconductor film through a gate insulating film.
(2) An MIS type thin film semiconductor device having a gate electrode which is formed below a predetermined polycrystalline semiconductor film through a gate insulating film.
(3) An MIS type thin film semiconductor device having a gate electrode which is formed on the side of a predetermined polycrystalline semiconductor film through a gate insulating film.
According to a second aspect of the present invention, there is provided a thin film semiconductor device which has an insulating substrate, a first semiconductor film which is a polycrystalline semiconductor film, a gate electrode formed on the first semiconductor film through a gate insulating film, first charge transmitting and receiving mean and second charge transmitting and receiving means formed on the first semiconductor film at a predetermined interval therebetween, and a channel region formed between the first and second charge transmitting and receiving means, wherein
the main orientation of the first semiconductor film constituting the channel region is {110} with respect to the main surface of the insulating substrate or the gate insulating film; and the first semiconductor film is essentially composed of crystal grains having an axis in a longitudinal direction of 45xc2x0 or less with respect to a direction for connecting the first and second charge transmitting and receiving means in the channel region. This thin film semiconductor device is a more practical embodiment of the present invention.
In the thin film semiconductor device of the present invention, the first semiconductor film comprises a small inclination grain boundary having an angle of 75xc2x0 or less with respect to a direction for connecting the first and second charge transmitting and receiving means.
According to a third aspect of the present invention, there is provided a thin film semiconductor device which has an insulating substrate, a first semiconductor film which is a polycrystalline semiconductor film, a gate electrode formed on the first semiconductor film through a gate insulating film, first charge transmitting and receiving mean and second charge transmitting and receiving means formed on the first semiconductor film at a predetermined interval therebetween, and a channel region formed between the first and second charge transmitting and receiving means, wherein
the main orientation of the first semiconductor film constituting the channel region is {110} with respect to the main surface of the insulating substrate or the gate insulating film; and the channel region of the first semiconductor film has crystal grains for connecting the first charge transmitting and receiving means and the second charge transmitting and receiving means.
The third aspect of the present invention is more preferred. That is, the channel region of the first semiconductor film is composed of crystal grains having such a length in a longitudinal direction as to connect the first charge transmitting and receiving means and the second charge transmitting and receiving means. Therefore, a thin film semiconductor device having higher reliability can be provided.
According to a fourth aspect of the present invention, there is provided a thin film semiconductor device which has an insulating substrate, a first semiconductor film which is a polycrystalline semiconductor film, a gate electrode formed on the first semiconductor film through a gate insulating film, first charge transmitting and receiving mean and second charge transmitting and receiving means formed on the first semiconductor film at a predetermined interval therebetween, and a channel region formed between the first and second charge transmitting and receiving means, wherein
the main orientation of the first semiconductor film constituting the channel region is {110} with respect to the main surface of the insulating substrate or the gate insulating film; and the main orientation of the surface of the first semiconductor film substantially perpendicular to a direction for connecting the first and second charge transmitting and receiving means is {100}. The channel region of the first semiconductor film is composed of crystal grains having a length in a longitudinal direction for connecting the first charge transmitting and receiving means and the second charge transmitting and receiving means, and the main orientation of the polycrystalline film is {100}, thereby making it possible to provide a thin film semiconductor device having extremely high reliability. Briefly speaking the conclusion, this is because each crystal grain has the same properties as a monocrystal though the first semiconductor film is polycrystal.
According to a fifth aspect of the present invention, there is provided a thin film semiconductor device which has at least two semiconductor device portions on an insulating substrate, a second semiconductor layer and an insulating film layer selectively formed on a partial region of the insulating substrate, the laminate consisting of the second semiconductor layer and the insulating film layer being in contact with a first semiconductor layer, the second semiconductor layer being not existent between a first semiconductor device and a second semiconductor device, a gate electrode formed on the first semiconductor layer through a gate insulating film, first charge transmitting and receiving means and second charge transmitting and receiving means formed on the first semiconductor film at a predetermined interval therebetween, and a channel region formed between the first and second charge transmitting and receiving means. Also in this thin film semiconductor device, the main orientation of the first semiconductor film constituting the channel region is more preferably {110} with respect to the main surface of the insulating non-crystalline substrate or the gate insulating film.
This thin film semiconductor device has the second semiconductor layer below a desired semiconductor device portion and the semiconductor device portion has the first aspect of the present invention. Further, in the present invention, the second semiconductor layer may be existent below the desired semiconductor device portion and the semiconductor device portion may have any one of the second to fourth aspects of the present invention. It is needless to say that the present invention can be carried out using any one of the semiconductor device portions disclosed in this specification.
According to a sixth aspect of the present invention, there is provided a thin film semiconductor device which has at least two semiconductor device portions on an insulating substrate, a second thin film which is selectively formed on a first thin film having a first heat conductivity and which has a second heat conductivity higher than the first heat conductivity, the second thin film being not in contact with a semiconductor layer and arranged between a first semiconductor device portion and a second semiconductor device portion, and the semiconductor device portions being the thin film semiconductor device of any one of the first to fourth aspects of the present invention.
According to a seventh aspect of the present invention, there is provided a thin film semiconductor device which has at least two semiconductor device portions on an insulating substrate, a second thin film which is selectively formed on a first thin film having a first heat conductivity and which has a second heat conductivity lower than the first heat conductivity, the second thin film being in contact with a semiconductor layer and not arranged between a first semiconductor device portion and a second semiconductor device portion, and the semiconductor device portions being the thin film semiconductor device of any one of the first to fourth aspects.
Practical variations of the thin film semiconductor device of the present invention will be described hereinafter.
A semiconductor film production process according to the major aspects of the present invention will be described hereinbelow. A polycrystalline semiconductor film of interest can be obtained by using the following process.
According to an eighth aspect of the present invention, there is provided a process for producing a polycrystalline semiconductor film by carrying out separately the step of growing crystal nuclei in a semiconductor layer formed on an insulating substrate and the step of melt recrystallizing a semiconductor thin film by irradiating a laser beam to grow crystal grains.
According to a ninth aspect of the present invention, there is provided a process for producing a polycrystalline semiconductor film, wherein a non-crystalline semiconductor film formed on the insulating substrate is melt recrystallized by irradiating a laser beam to grow crystal grains.
According to a tenth aspect of the present invention, there is provided a process for producing a polycrystalline semiconductor film, wherein the wavelength of the laser beam is selected from a range of 240 nm to 600 nm and the absorption coefficient for the wavelength of the non-crystalline semiconductor film is larger than the absorption coefficient of the polycrystalline semiconductor film.
According to an eleventh aspect of the present invention, there is provided a process for producing a polycrystalline semiconductor film, wherein the crystal nuclei of the polycrystalline silicon film formed on the insulating film are formed by melt recrystallization by the irradiation of a laser beam.
The crystal nuclei of the polycrystalline silicon film on the insulating substrate are preferably grown by catalytic chemical vapor deposition.
The production apparatus of the present invention comprises means for modulating the pulse width of a laser beam, the time-dependent shape of laser beam intensity and the interval of laser beam pulses, means for shaping the irradiation range of the profile of the laser beam irradiated from an oscillation source to a predetermined form and focusing it on an object to be irradiated, and means of moving the insulating substrate at a predetermined speed and pitch in synchronism with the irradiation of the laser beam.
In the present invention, the charge transmitting and receiving means generally means a source or drain region.